Decorrelating Ambiguities in SAR Interferometry Through Slight PRI Variation

Synthetic aperture radar (SAR) interferometry is a well-established technique for producing high-resolution digital elevation models (DEMs) of the Earth's surface and measuring displacements on different time scales. Observations of SAR interferograms, however, show that azimuth ambiguities can be coherently imaged and may lead to phase biases and coherence losses that significantly degrade the interferometric performance. Whereas imposing very low ambiguity levels may represent a severe design constraint for a spaceborne SAR system, a slight variation of the pulse repetition interval (PRI) is a new, simple, yet effective technique to decorrelate ambiguities, which, in turn, reduces the phase biases and coherence losses without substantially affecting the imaged swath width. An additional benefit of the PRI variation is that range ambiguities also become decorrelated. This article addresses two cases. For the repeat-pass case, slightly different pulse repetition frequencies (PRFs) can be used for the two acquisitions, and the minimum required PRF difference can be analytically derived resorting to the power spectral density of the ambiguous signals. For the single-pass case, a slight variation of the PRI during the common acquisition is an effective solution in case an along-track baseline is present. In particular, a square wave PRI variation scheme outperforms sinusoidal or random ones. Finally, simulations using TanDEM-X data are presented to show the improvement in interferogram and DEM quality resulting from ambiguity decorrelation. This work is relevant for the design of future spaceborne interferometric SAR systems and for the enhanced exploitation of current ones.